<!Doctype html>
<html>
    <head>
        <meta charset='utf-8'/>
        <title>CSS画三角形</title>
        <style type='text/css'>
            body{
                background-color: pink;
            }
            .box{
                width:0px;
                height:0px;
                border:100px solid;
                border-top-color: red;
                
                /*不要换成白色*/
                border-bottom-color: transparent;
                border-left-color:transparent;
                border-right-color:transparent ;

            }
        </style>
    </head>
    <body>
        <div class='box'></div>
        <script type='text/javascript'>
  
        </script>
    </body>
</html>